Question 30

[nsross]

I originally chose II as my answer and changed it upon second glance-- II is a 9-C ring, and derivative or not, it's certainly not cyclohexane...

[admin]

Good question.

To begin with, what is a derivative in organic chemistry? A derivative is a compound derived or obtained from another compound and containing essential elements of the parent substance. In the end, the 2 chemicals can have quite a number of differences including and especially the number of carbons (whether or not those carbons form an additional ring as in this particular question).

Consider the following carboxylic acid derivatives which you are responsible to know for the MCAT: acid halides, acid anhydrides, amides, esters and beta keto acids (Chap 9). Now if you look at the derivatives and compare to the parent compound (carboxylic acid), you will notice many differences (as you know, amides are not even acids and the anhydride is roughly double the size of the parent compound). In other words, because II can be produced using cyclohexane as a parent compound, then it is definitely a derivative of cyclohexane (of course, you are not expected to know the mechanism but rather only to notice the common aspects of the molecules).

Besides the above, the logic in the Explanation to the answer does not leave room for any other option.

[kazuma]

hmm, wouldn't 1 be locked as well, bc its t-butyl is in the equatorial position? this is how I took the question; the 2 chair conformations of cyclohexane can differ in their energies, if these energies are great enough, it will maintain itself in one coformation (the one with the lowest energy). Wouldn't it be nearly impossible for that bulky substituent to go into the axial position, thus making the chair 'locked'?

[admin]

From the question:

[bravozullu4764]

the previous post about 1 being too stable to change conformation is correct. If you're going to argue that that's possible, then it's possible that II can change conformation. bad question

[maxsati7049]

I understand that cyclohexane is naturally present in equilibrium and changes its chair confirmation continuously. However, I thought if you slap a bulky group on a cyclohexane then the energy required to change the position of that bulky group (whether it was on the axial or equatorial position) is much too large and the ring will not do it. Hence ring locked...

[jellywing_2058]

Structure II is ring locked because it can only have one chair conformation. If you build the molecule with a molecular kit, you will clearly see this. The boat conformation is of high energy and is only used for the transition from one chair conformation to the other; it does not count as a structural isomer.

[quadalpha]